Place the tomatoes in a colander set over a bowl and refrigerate overnight to drain excess water. Combine tomatoes, onions, capsicum and garlic in a large pot and cook over medium heat till soft.

Transfer to a blender, blend till smooth and strain through a sieve into a clean pot. Tie the spices in a square of muslin and add to the tomato puree. Simmer, stirring frequently, over medium heat till mixture is thick, about 1 hour.

Add the sugar, vinegar, chilli powder and salt, continue to cook till thickened. Remove spice bag from ketchup. Cool completely before bottling. Ketchup will keep in the refrigerator up to 6 months.